PROJECT SUMMARY/ABSTRACT ? Project 1: Optimizing Treatment of GBM by FLASH We are proposing the creation of a research program entitled, ?Increasing the therapeutic index of brain tumor treatment through innovative FLASH radiotherapy (FLASH-RT),? focused on translating a novel irradiation modality rapidly into the clinic. The overall hypothesis to be tested is that radiation delivered at ultra-high dose rates (compared to the much lower dose rates used in current clinical practice) can significantly ameliorate normal tissue complications while maintaining acceptable if not improved tumor control. To test this hypothesis, the program will deploy a comprehensive series of preclinical studies that will critically evaluate tumor control, neurocognitive outcomes and resultant radiation injury to the brain following FLASH-RT and conventional dose rate irradiation. Collectively, these studies will generate the requisite data sets required for the rapid translation of the novel FLASH irradiation platform to the clinical setting. Preclinical studies in mice assessing orthotopic tumor control, cognition, neuronal and vascular structure, immunomodulation, and oxygen-dependent mechanisms of radiation injury are coupled with a clinical trial in GBM dog patients to inform the oncologists of the potential benefits of this potentially paradigm shifting approach. The objectives of this program project will be facilitated by the activities conducted by the Dosimetry/Physics/Modeling core and the Neurobehavioral core. Project 1 will focus on the assessing the therapeutic index, comprising tumor control and normal brain injury, of FLASH-RT compared to conventional dose rate irradiation. Orthotopic tumor-bearing mice will be treated with FLASH and conventional RT under clinically relevant scenarios: single fraction and fractionated, with and without concurrent chemotherapy, using electron and MV x-ray beams. The experiments will be conducted across our institutions (CHUV, Stanford, and Indiana University) using our novel FLASH irradiation platforms, supported by Cores 2 and 3. We will evaluate cellular and molecular mechanisms underlying the differential effects of FLASH by assessing neuronal and tumor structure, and markers of inflammation/immunomodulation. The success of this innovative program project grant is bolstered by the unparalleled breadth and depth of our multi-disciplinary investigative team at UC Irvine, Stanford University, SLAC National Accelerator Laboratory, CHUV/Lausanne University Hospital, and Indiana University that has pioneered the development of the initial experimental infrastructure for conducting FLASH-RT research and produced strong preclinical evidence of increased therapeutic index, comprising expertise in radiation oncology, radiobiology, medical physics, and preclinical imaging and accelerator science. In summary, Project 1 will pave the way for near-term clinical translation of FLASH-RT by systematically testing FLASH and conventional dose-rate irradiation under clinically relevant regimens, and will complement studies in Projects 2-4 focused on longer-term neurotoxicity, expanding to other species preclinically, and physico-chemical mechanisms of FLASH-RT.